Maeve was 8 lbs. and 21½ inches long when she was born Dec. 10 in Los Angeles. She joins sister Lily, 8, and brothers Chip, 7; Charlie, 4; Finley, 1.

Grey's Anatomy alum O'Donnell, 37, who starred in the Golden Globe-nominated TNT miniseries The Company, broke the news of his growing brood in August, joking that he and his wife were "just starting to figure out how it works."

Next up, he stars in Kit Kittredge: An American Girl, due out next year, playing a role he's becoming increasingly familiar with: a dad.